Covalent bonding occurs when two or more nonmetals share electrons to attain a stable octet. The shared electron pairs may be distributed unequally between the atoms. The concept of electronegativity describes an atom's ability to attract shared electrons towards itself. Atoms with a large difference in electronegativity form ionic bonds, while smaller differences result in polar or nonpolar covalent bonds. Electronegativity values provide an estimate of bond polarity.

1. Covalent Bonding
Many substances do not have the characteristics of ionic materials.
The atoms of covalent substances acquire a noble gas electron configuration by sharing
electrons with other atoms.
Covalent bonding occurs when two or more nonmetals share electrons, attempting to attain a
stable octet of electrons.
Bond Polarities and Electronegativity
The electron pairs shared between two atoms are not necessarily shared equally.
We can visualize two extreme cases in the degree to which electrons are shared.
On the one hand, we have bonding between two identical atoms, as in Cl2 or N2 where the
electrons must be shared equally.
At the other extreme, we have essentially no sharing of electrons, as illustrated by NaCl.

2. The bonding occurring in most covalent substances fall somewhere between these two
extremes.
In practice we describe bonds as either ionic or covalent, depending on what extreme the
bond more closely resembles.
The concept of bond polarity is useful in describing the sharing of electrons.
Nonpolar (pure covalent) - electrons are shared equally.
Polar covalent- electrons are shared unequally.
Ionic - difference in ability to attract electrons is very large.
We use a quantity called electronegativity to estimate whether a given bond will be nonpolar,
polar covalent, or ionic.
Electronegativity is defined as the ability of an atom in a molecule to attract electrons to
itself.
The greater an atom's electronegativity, the greater it's ability to attract electrons to
itself.

3. American chemist Linus Pauling first developed the concept of electronegativity.
He based his scale on bond- energy relationships.
We will not be concerned with how the electronegativity values are obtained, but rather with
using the concept in discussing chemical bonding.
Keep in mind that electronegativities are approximate measures of the relative tendencies
of the atoms of elements to attract electrons to themselves in a chemical bond.
We can use the difference in electronegativity between two atoms to gauge the polarity of
the bonding between them.
Consider the fluorine- containing compounds listed below:
Compounds F2 HF LiF
Electronegativity 0 1.8 3.0
Difference
Type of Bond non polar polar ionic

4. In F2, the electrons are shared equally between the fluorine atom, and the bond is nonpolar.
In HF, the fluorine atom has a greater electronegativity than the hydrogen atom.
Consequently the sharing of electrons is unequal (the bond is polar)
We represent the sharing of the electrons in the following two ways.
-
+
H-F or δ+
H-F δ-
+ and - are meant to the represent partial positive and partial negative charges
+ -
respectively.
In LiF, the far greater electronegativity of fluorine as compared to Lithium leads to the
complete transfer of valence electrons off Li to F.
This transfer results in the formation of Li+ and F- ions, the resultant bond is therefore
ionic.
Remember: the greater the electronegativity difference, the more polar the bond.

5. Electronegativity Bond Type
Difference
0- 0.5 Nonpolar
0.51-2.0 Polar
2.1 - above Ionic